u/debug_gui: Do not call gui_prober_update in runtime debug gui

The calling of this function is only needed for the debug gui used in
monado-gui target, not the one built into the runtime. This better simulates
drivers not getting called while no apps are connected.

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2399>

src/xrt/auxiliary/util/u_debug_gui.c

@@ -289,9 +289,6 @@ sdl2_loop(struct u_debug_gui *p)
 		XRT_TRACE_BEGIN(swap);
 		SDL_GL_SwapWindow(p->win);
 		XRT_TRACE_END(swap);
-
-		// Update prober things.
-		gui_prober_update(&p->base);
 	}
 
 	// Cleanup